Resizing is not some magical process that you can perform over and over and get better results each time.
At a certain point it reaches a limit, based on the settings you've selected. You should do some research and find out what elements take up the most place in your file (open the Content pane on the right, right-click the top-most level item and select Audit Space Usage). Is it images? Fonts? Content Streams? Forms? Something else?
It's possible to downgrade images, for example, to get a smaller file-size (in exchange for visual quality, of course), or to remove unnecessary fonts, etc.
But it's not reasonable to expect an ever-smaller file size just because you run the same tool over and over.
The document is made up of text only, so visual quality does not have to be very good. I'm just surprised that the smallest size I can get that document down to is only 18 megs. I am hoping there is another way to reduce the file size more. I tried converting the pages to JPG but they saved in a random order, and would require work repositioning the order of the pages to make it linear. My thought would be that I would then convert them back to PDF at a much lower rez. I should add that the document is made up of 78 scanned pages.
Check the Space Usage as I've described. My guess is you have all kinds of fonts embedded in the file...